Process for manufacturing high purity methacrylic acid

ABSTRACT

A process is provided herein for the high yield production of high purity glacial methacrylic acid (“HPMAA”) that is substantially pure, specifically 99% pure or greater with a water content of 0.05% or less. This improved process comprises providing a crude MAA stream and purifying the crude methacrylic acid stream in a series of successive distillation steps

The present invention is related to a process for the production ofsubstantially pure methacrylic acid that is at least 99% pure furtherhaving 0.05% or less water content.

Methacrylic acid (“MAA”) and methacrylate esters such as methylmethacrylate (“MMA”) are used in a wide variety of applications. Typicalend-use applications include: acrylic plastic sheeting; molding resins;polyvinyl chloride modifiers; processing aids; acrylic lacquers; floorpolishes; sealants; auto transmission fluids; crankcase oil modifiers;automotive coatings; ion exchange resins; cement modifiers; watertreatment polymers; electronic adhesives; metal coatings; and acrylicfibers. MAA and methacrylate esters are especially prized in theseapplications and others because of the hardness they impart to theproducts in which they are used. They also enhance chemical stabilityand light stability, as well as ultraviolet radiation resistance, of theproducts in which they are used. Therefore, MAA and methacrylate estersare often used in applications requiring resins of excellenttransparency, strength, and outdoor durability. The MAA market isextremely cost-sensitive; thus, any improvement in process yield,however slight, can result in significant market advantage.

MAA that has a very low percentage by weight of impurities is verydesirable. MAA having an impurity level of less than 5% by weight isreferred to herein as glacial methacrylic acid (“GMAA”). MAA having animpurity level of less than 1% by weight wherein no more than 0.05% byweight is water is referred to herein as high purity glacial methacrylicacid (“HPMAA”).

A conventional process for purifying MAA involves treating the streamwith an excess of sulfuric acid to remove some of the water. While thisprocess yields MAA of sufficient purity for use in batch production ofdirect-esterification esters, it also creates undesirable sulfur-bearingresidue. Moreover, the MAA is not purified to 95%, and, therefore, doesnot meet the requirements of a GMAA or an HPMAA product. Further, suchconventional processes for the removal of water from MAA have resultedin increased yield losses and additional waste. Thus, specifications forGMAA (i.e., MAA that is at least 95% pure) and HPMAA (i.e., MAA that is99% pure further having less than 0.05% water content) have not been metwith conventional purification processes.

Therefore, there is an unaddressed need for a method to produce HPMAA ata reduced cost for the manufacturer. Further, there is an unaddressedneed to produce HPMAA that consistently meets the product specificationsof at least 99% purity and less than 0.5% water.

The present invention solves the problems inherent in the prior art byproviding an economical method for producing HPMAA. The presentinvention involves the purification of MAA to HPMAA via the use ofdistillation columns to remove various impurities (generally, lightimpurities, heavy impurities, and water) from an MAA stream such thatthe resultant product is at least 99% pure MAA with not more than 0.05%water.

Thus, provided herein is a process for the preparation of high purityglacial methacrylic acid, said process comprising:

-   -   (i) providing a first distillation column, a second distillation        column and a third distillation column;    -   (ii) feeding a crude methacrylic acid to an upper section of        said first distillation column, said crude methacrylic acid        comprising methacrylic acid, light ends and heavy ends;    -   (iii) distilling said crude methacrylic acid in said first        distillation column to form a first overhead vapor stream        comprising light ends and a first bottom liquid stream        comprising methacrylic acid and heavy ends;    -   (iv) feeding said first bottom liquid stream to a center section        of said second distillation column;    -   (v) distilling said first bottom liquid stream in said second        distillation column to form a second overhead vapor stream        comprising methacrylic acid and light ends and a second bottom        liquid stream comprising heavy ends;    -   (vi) feeding at least a portion of said second overhead vapor        stream to an upper section of said third distillation column;    -   (vii) distilling said at least a portion of said second overhead        vapor stream in said third distillation column to form a third        overhead vapor stream comprising light ends and a third bottom        liquid stream comprising methacrylic acid wherein said        methacrylic acid has an impurity level of not more than 1% by        weight wherein no more than 0.05% by weight is water.

Additionally, provided herein is a further process for the preparationof high purity glacial methacrylic acid, said process comprising:

-   -   (i) providing a first distillation column, a second distillation        column and a third distillation column;    -   (ii) feeding a crude methacrylic acid to an upper section of        said first distillation column, said crude methacrylic acid        comprising methacrylic acid, light ends and heavy ends;    -   (iii) distilling said crude methacrylic acid in said first        distillation column to form a first overhead vapor stream        comprising light ends and a first bottom liquid stream        comprising methacrylic acid and heavy ends;    -   (iv) feeding said first bottom liquid stream to a center section        of said second distillation column;    -   (v) distilling said first bottom liquid stream in said second        distillation column to form a second overhead vapor stream        comprising methacrylic acid and light ends and a second bottom        liquid stream comprising heavy ends;    -   (vi) feeding at least a portion of said second overhead vapor        stream to an upper section of said third distillation column;    -   (vii) distilling said at least a portion of said second overhead        vapor stream in said third distillation column to form a third        overhead vapor stream comprising light ends and a third bottom        liquid stream comprising heavy ends, while withdrawing a first        liquid sidestream from a lower section of said third        distillation column, said first liquid sidestream comprising        methacrylic acid wherein said methacrylic acid has an impurity        level of not more than 1% by weight wherein no more than 0.05%        by weight is water;    -   (viii) feeding said third bottom liquid stream to said center        section of said second distillation column.

Additionally, provided herein is a still further process for thepreparation of high purity glacial methacrylic acid, said processcomprising:

-   -   (i) providing a first distillation column, a second distillation        column and a third distillation column;    -   (ii) feeding a crude methacrylic acid to a center section of        said first distillation column, said crude methacrylic acid        comprising methacrylic acid, light ends and heavy ends;    -   (iii) distilling said crude methacrylic acid in said first        distillation column to form a first overhead vapor stream        comprising methacrylic acid and light ends and a first bottom        liquid stream comprising heavy ends;    -   (iv) feeding at least a portion of said first overhead vapor        stream to an upper section of said second distillation column;    -   (v) distilling said at least a portion of said first overhead        vapor stream in said second distillation column to form a second        overhead vapor stream comprising light ends and a second bottom        liquid stream comprising methacrylic acid and heavy ends;    -   (vi) feeding said second bottom liquid stream to a center        section of said third distillation column;    -   (vii) distilling said second bottom liquid stream in said third        distillation column to form a third overhead vapor stream        comprising methacrylic acid wherein said methacrylic acid has an        impurity level of not more than 1% by weight wherein no more        than 0.05% by weight is water and a third bottom liquid stream        comprising heavy ends;    -   (viii) feeding at least a portion of said third overhead vapor        stream to an upper section of said third distillation column;    -   (ix) feeding said third bottom liquid stream to said center        section of said first distillation column.

Moreover, the present invention also provides an apparatus for thepreparation of high purity glacial methacrylic acid, said apparatuscomprising:

-   -   (i) a first distillation column, said first distillation column        having a top, a bottom, an upper section adjacent said top, a        lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (ii) a second distillation column, said second distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iii) a third distillation column, said third distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iv) a first inlet line connected to said upper section of said        first distillation column;    -   (v) a first outlet line connected to said top of said first        distillation column;    -   (vi) a second outlet line connecting said bottom of said first        distillation column and said center section of said second        distillation column;    -   (vii) a third outlet line connecting said top of said second        distillation column with said upper section of said second        distillation column and said upper section of said third        distillation column;    -   (viii) a fourth outlet line connected to said bottom of said        second distillation column;    -   (ix) a fifth outlet line connected to said top of said third        distillation column;    -   (x) a sixth outlet line connected to said bottom of said third        distillation column.

Additionally, the present invention further provides an apparatus forthe preparation of high purity glacial methacrylic acid, said apparatuscomprising:

-   -   (i) a first distillation column, said first distillation column        having a top, a bottom, an upper section adjacent said top, a        lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (ii) a second distillation column, said second distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iii) a third distillation column, said third distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iv) a first inlet line connected to said upper section of said        first distillation column;    -   (v) a first outlet line connected to said top of said first        distillation column;    -   (vi) a second outlet line connecting said bottom of said first        distillation column and said center section of said second        distillation column;    -   (vii) a third outlet line connecting said top of said second        distillation column with said upper section of said second        distillation column and said upper section of said third        distillation column;    -   (viii) a fourth outlet line connected to said bottom of said        second distillation column;    -   (ix) a fifth outlet line connected to said top of said third        distillation column;    -   (x) a sixth outlet line connected to said lower section of said        third distillation column;    -   (xi) a seventh outlet line connecting said bottom of said third        distillation column to said center section of said second        distillation column.

Additionally, the present invention still further provides an apparatusfor the preparation of high purity glacial methacrylic acid, saidapparatus comprising:

-   -   (i) a first distillation column, said first distillation column        having a top, a bottom, an upper section adjacent said top, a        lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (ii) a second distillation column, said second distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iii) a third distillation column, said third distillation        column having a top, a bottom, an upper section adjacent said        top, a lower section adjacent said bottom and a center section        intermediate said upper section and said lower section;    -   (iv) a first inlet line connected to said center section of said        first distillation column;    -   (v) a first outlet line connecting said top of said first        distillation column with said upper section of said first        distillation column and said upper section of said second        distillation column;    -   (vi) a second outlet line connected to said bottom of said first        distillation column;    -   (vii) a third outlet line connected to said top of said second        distillation column;    -   (viii) a fourth outlet line connecting said bottom of said        second distillation column with said center section of said        third distillation column;    -   (ix) a fifth outlet line connecting said top of said third        distillation column with said upper section of said third        distillation column and with a product withdrawal line;    -   (x) a sixth outlet line connecting said bottom of said third        distillation column with said center section of said first        distillation column.

For clarity, the following definitions are used herein: “top” is thevapor space existing at the extreme top of a distillation column;“bottom” is the liquid sump existing at the extreme bottom of adistillation column; “upper section” is the approximate uppermost ⅓ ofthe distillation column which is below and adjacent to the “top”; “lowersection” is the approximate lowermost ⅓ of the distillation column whichis above and adjacent to the “bottom”; “center section” is theapproximate ⅓ of the distillation column intermediate the “uppersection” and the “lower section”; “line” is a fluidic connection fortransporting vapor and/or liquid into a unit, out of a unit or betweentwo or more units, and may include such common peripherals as valves,condensers, flow meters, etc.

Other and further objects, features and advantages will be apparent fromthe following description of some embodiments of the invention. Theseembodiments are given for the purpose of disclosure and may beconsidered in conjunction with the accompanying drawings.

A more complete understanding of the present embodiments and advantagesthereof may be acquired by referring to the following description takenin conjunction with the accompanying drawings, in which like referencenumbers indicate like features, and wherein:

FIG. 1 is a diagram of one embodiment of the present invention of aprocess for producing HPMAA.

FIG. 2 is a diagram of an alternative embodiment of the presentinvention of a process for producing HPMAA.

FIG. 3 is a diagram of another embodiment of the present invention of aprocess for producing HPMAA.

The present invention involves methods of purifying methacrylic acid(“MAA”) to a high-purity methacrylic acid (“HPMAA”). HPMAA is defined ashaving less than 1% impurities and no more than 0.05% water. The MAAfeedstock that could be beneficially purified in the methods of thepresent invention may be derived from any available industrial process;examples of such process include but are not limited to, anisobutane/isobutylene-based process, an ethylene-based process, or aproplyene-based process. One example of a proplyene-based process forthe production of MAA involves an acetone cyanohydrin (“ACH”) feedstockbeing subjected to the steps of hydrolysis, cracking, reaction, andseparation. In such a MAA production process, ACH is reacted with anexcess of sulfuric acid to hydrolyze the ACH. Then, the hydrolyzedmixture is cracked in a cracking train and then reacted with water toform crude MAA. Finally, the crude MAA stream is then cooled and allowedto separate (essentially a buoyancy separation) into an MAA productstream and a lower layer sulfur-bearing residue stream. The MAA productsteam is then purified via the methods of the present invention.

In one embodiment of the method of the present invention to produceHPMAA, shown in FIG. 1, a crude MAA stream is provided via line 100 tothe first of three impurity removal apparatus, HPMAA lights column 110.In HPMAA lights column 110, light ends such as acetone and water areremoved via line 105. HPMAA lights column 110 also includes columnancillaries, wherein the term “column ancillaries” means any and allsecondary equipment and associated piping that is connected to a column,such as vacuum equipment, reboilers, condensers, pumps, and processlines including but not limited to feed lines, bottoms lines, overheadlines, vent lines, inhibitor addition lines, oxygen addition lines,reflux lines, and rundown lines.

HPMAA lights column 110 and its column ancillaries are preferablyconstructed of materials resistant to corrosion. Suitable materials ofconstruction resistant to corrosive effects include but are not limitedto: 300 series stainless steel, 904L, 6-moly stainless steel, HASTELLOY®(e.g., B, B-2, B-3, C-22, and C-276), tantalum, and zirconium. In someembodiments, the manufacturer may reduce construction costs by utilizingcovered base metals. “Covered base metal” materials are materials thatare generally thought not to be corrosion resistant, such as carbonsteel, combined with a covering capable of resisting corrosion such asglass, epoxy, elastomer, fluoropolymer (e.g., TEFLON®), or one of theabove-listed corrosion resistant metals. Covered base metals areconstructed by placing a covering capable of resisting corrosion over,and optionally bonding the covering to, the base metal. The coveringprevents contact between the base-metal and the process stream. Coveredbase-metal construction is especially preferred for large-diameterpiping (3.8 cm or larger nominal diameter) and for heat exchanger tubesin high fluid-velocity service (fluid velocity of 0.15 meter/second ormore) and other components, where significant metal thickness (3 mm ormore metal thickness) may be used to provide structural strength. Thematerials described above such as 300 series stainless steel, 904L,6-moly stainless steel, HASTELLOY® (e.g., B, B-2, B-3, C-22, and C-276),tantalum, zirconium, and covered base-metal materials are hereinafterreferred to as “corrosion resistant material.”

Internal components such as trays or packing may be used in HPMAA lightscolumn 110, if desired. Internals, if present, may be made from the samematerials as the column itself or may be constructed from one or moredifferent materials; for example, the upper portion of the column maycontain 300 series stainless steel packing, while the lower portion ofthe column contains HASTELLOY® B-2 packing. Trays are preferred for usein HPMAA lights column 110. Perforated plate trays are especiallypreferred, as they have been found to be particularly resistant to MAApolymer accumulation. By the term “perforated plate trays” as usedherein is meant any tray comprising a planar portion with a plurality ofholes through said planar portion. Optional tray features, including butnot limited to weirs, downcomers, baffles, distributors, valves,bubblecaps, and drain holes, may also be present. Examples of perforatedplate trays include sieve trays, dualflow trays, and combinationvalve+perforation trays. If trays are used, it is preferable that two toten perforated plate trays be used.

It is also preferred that HPMAA lights column 110 be operated under avacuum to minimize the temperature at the bottom of the column. Forexample, in a preferred embodiment, the pressure at the bottom of thecolumn is maintained from 50 mmHg to 80 mmHg, allowing the bottom of thecolumn to be operated from 70° C. to 110° C.

At least one heat exchanger may be used as the heating apparatus forHPMAA lights column 110. Desuperheated steam is the preferred heatsource for these exchangers. If a reboiler is used as the heatexchanger, it may be internal or external to the distillation column.Vortex breakers are also useful in the bottom of HPMAA lights column110.

It is oftentimes useful to add water-soluble or alcohol-solublepolymerization inhibitor to HPMAA lights column 110. Suitable examplesinclude but are not limited to:

-   Hydroquinone (HQ);-   4-methoxyphenol (MEHQ);-   4-ethoxyphenol;-   4-propoxyphenol;-   4-butoxyphenol;-   4-heptoxyphenol;-   hydroquinone monobenzylether;-   1,2-dihydroxybenzene;-   2-methoxyphenol;-   2,5-dichlorohydroquinone;-   2,5-di-tert-butylhydroquinone;-   2-acetylhydroquinone; hydroquinone monobenzoate;-   1,4-dimercaptobenzene;-   1,2-dimercaptobenzene;-   2,3,5-trimethylhydroquinone;-   4-aminophenol;-   2-aminophenol;-   2-N,N-dimethylaminophenol;-   2-mercaptophenol;-   4-mercaptophenol; catechol monobutylether;-   4-ethylaminophenol;-   2,3-dihydroxyacetophenone;-   pyrogallol-1,2-dimethylether;-   2-methylthiophenol;-   t-butyl catechol;-   di-tert-butylnitroxide;-   di-tert-amyInitroxide;-   2,2,6,6-tetramethyl-piperidinyloxy;-   4-hydroxy-2,2,6,6-tetramethyl-piperidinyloxy;-   4-oxo-2,2,6,6-tetramethyl-piperidinyloxy;-   4-dimethylamino-2,2,6,6-tetramethyl-piperidinyloxy;-   4-amino-2,2,6,6-tetramethyl-piperidinyloxy;-   4-ethanoloxy-2,2,6,6-tetramethyl-piperidinyloxy;-   2,2,5,5-tetramethyl-pyrrolidinyloxy;-   3-amino-2,2,5,5-tetramethyl-pyrrolidinyloxy;-   2,2,5,5-tetramethyl-1-oxa-3-azacyclopentyl-3-oxy;-   2,2,5,5-tetramethyl-3-pyrrolinyl-1-oxy-3-carboxylic acid;-   2,2,3,3,5,5,6,6-octamethyl-1,4-diazacyclohexyl-1,4-dioxy;-   salts of 4-nitrosophenolate;-   2-nitrosophenol;-   4-nitrosophenol;-   copper dimethyldithiocarbamate;-   copper diethyldithiocarbamate;-   copper dibutyldithiocarbamate;-   copper salicylate;-   methylene blue;-   iron;-   phenothiazine (PTZ);-   3-oxophenothiazine;-   5-oxophenothiazine;-   phenothiazine dimer;-   1,4-benzenediamine;-   N-(1,4-dimethylpentyl)-N′-phenyl-1,4-benzenediamine;-   N-(1,3-dimethylbutyl)-N′-phenyl-1,4-benzenediamine;-   N-nitrosophenyl hydroxylamine and salts thereof;-   nitric oxide;-   nitrosobenzene;-   p-benzoquinone; or    isomers thereof; mixtures of two or more thereof; mixtures of one or    more of the above with molecular oxygen. The inhibitor(s) may be    used alone or combined with a suitable diluent. Preferred diluents    include, but are not limited to, MAA, water, and organic mixtures    comprising acetone.

Hydroquinone (“HQ”) inhibitor is especially preferred for use in HPMAAlights column 110, and it may be added directly, or with a diluent inone or more locations throughout HPMAA lights column 110 and itsancillaries. If used, it is preferred that the inhibitor be added at arate of 1 kg to 10 kg of HQ per 10,000 kg of HPMAA lights column feed;more preferably 1.3 kg to 8 kg of HQ per 10,000 kg of HPMAA lightscolumn feed; most preferably 1.5 kg to 5 kg of HQ per 10,000 kg of HPMAAlights column feed.

When phenolic inhibitors, such as HQ and MeHQ, are used, it is furtherpreferred that oxygen-containing gas be added to the distillation columnto enhance the effectiveness of the inhibitor. The term“oxygen-containing gas,” as used herein, refers to any gas comprisingfrom 0.01% up to 100% oxygen. Oxygen-containing gas may be added in oneor more locations throughout HPMAA lights column 110 and its columnancillaries. Operating temperatures and pressures impact theflammability limits and oxygen solubility within the purificationsystem, and these properties must be taken into account when determiningthe appropriate oxygen concentration to be used for theoxygen-containing gas. Considerations of such factors are within theability of one of ordinary skill in the art, and either pure oxygen oratmospheric air may be commonly employed. Surprisingly, we have foundthat there is an important factor affecting the efficacy of inhibitionwithin the purification systems not previously considered with respectto oxygen addition—that is the avoidance of high oxygen concentrationswithin the monomer-containing solution itself. When oxygenconcentrations are large relative to inhibitor concentrations, oxygencan actually increase the rate of polymerization by promoting theformation of monomer radicals. For this reason, it is not recommendedthat oxygen-containing gas be added when no inhibitor is present.Further, it is preferred that when oxygen-containing gas and inhibitorsare added to the purification system, that the oxygen-containing gas beadded in a prescribed ratio with respect to the inhibitor addition rate.The optimal oxygen to inhibitor ratio will vary with respect to theinhibitor used. When HQ is the selected inhibitor, it is preferred thatthe ratio of the oxygen-containing gas feed to the HQ inhibitor feedadded to the purification system is maintained at 0.65 moles to 10 molesof O₂/mole of HQ; more preferably at 1 moles to 8.5 moles of O₂/mole ofHQ; most preferably at 1.5 moles to 6 moles of O₂/mole of HQ. When MEHQis the selected inhibitor, it is preferred that the ratio ofoxygen-containing gas feed to the MEHQ inhibitor feed added to thepurification system be maintained at 1 moles to 11.5 moles of O₂/mole ofMEHQ; more preferably at 1.5 moles to 9 moles of O₂/mole of MEHQ; mostpreferably at 2 moles to 6 moles of O₂/mole of MEHQ.

The light ends such as acetone and water, along with some MAA, are takenoff of the top of HPMAA lights column 110 via line 105; this stream maybe recycled for use elsewhere (e.g., in the MAA process) or may berouted to an acetone recovery vessel. To minimize condensationpolymerization, vapor spaces on HPMAA lights column 110 and itsancillaries, including condensers and interconnecting vapor lines, arepreferably maintained at a temperature above the dewpoint of MAA;insulation and electric or steam tracing are effective for this purpose.

If stream 105 is condensed after removal from HPMAA lights column 110,coolant having a temperature above 16° C. may be used in the condenserto avoid freezing MAA in the stream. In a preferred embodiment, temperedwater in the range of 16° C. to 35° C. is used for the condensercoolant. In one embodiment, a portion of the condensate may berecirculated back to the condenser(s) and optionally to the vapor inletline, to minimize fouling and improve condenser efficiency. Thecondensate may flow freely out of the recirculation line or may beapplied to the tubesheet, condenser interior surfaces, and/or inletvapor line interior walls. If inhibitor is added to the condenser(s), itmay be added through this condensate recirculation stream to improve thedistribution of the inhibitor. In an especially preferred embodiment, atleast a portion of this condensate recirculation stream may pass throughan apparatus that sprays the condensate on the interior surfaces ofHPMAA lights column 110 and/or its ancillaries to wash off polymerizablecondensates.

In another embodiment, a partial-condenser arrangement is utilized,wherein stream 105 is divided into two or more streams, including atleast one MAA/water stream and one water/acetone stream. In this way,the MAA/water stream can be recycled directly into an MAA process andthe water/acetone stream can be routed to another process such as anacetone recovery operation, a scrubber, or a flare.

HPMAA lights column 110 bottoms stream 115 contains MAA and heavy ends,such as hydroxy isobutyric acid (“HIBA”), and is substantially free ofacetone and water. Stream 115 is fed to the second impurity removalapparatus, HPMAA heavies column 120. In HPMAA heavies column 120, theheavy impurities produced in the crude MAA production process areseparated from the MAA. It is particularly important to remove HIBA, asremaining HIBA will tend to decompose in later processing steps. HPMAAheavies column 120 and its column ancillaries are preferably constructedof corrosion resistant material, as described above for HPMAA lightscolumn 110.

Internal components such as trays or packing may be used in HPMAAheavies column 120, if desired. Internals, if present, may be made fromthe same materials as the column itself or may be constructed from oneor more different materials; for example, the upper portion of HPMAAheavies column 120 may contain 300 series stainless steel trays, whilethe lower portion of the column contains 904L trays. Trays are preferredin HPMAA heavies column 120. Perforated plate trays are especiallypreferred, as they have been found to be particularly resistant to MAApolymer accumulation. If trays are used, it is preferable that five tofifteen perforated plate trays are used. It is preferred that theheavies column 120 be operated under a vacuum to minimize the bottomstemperature. For example, in a preferred embodiment, the pressure at thebottom of the column be maintained at 60 mmHg to 100 mmHg, allowing thebottom of the column to be operated at 75° C. to 115° C.

At least one heat exchanger may be used as the heating apparatus forHPMAA heavies column 120. Desuperheated steam is preferred as the heatexchanger's heat source. If a reboiler is used as the heat exchanger, itmay be internal or external to the column. Vortex breakers are alsouseful in the bottom of column 120.

It is oftentimes useful to add one or more inhibitors, such as thoselisted above, to HPMAA heavies column 120 with or without a diluent.MEHQ is a preferred inhibitor and may be added directly, or with adiluent such as MAA, in one or more locations throughout HPMAA heaviescolumn 120 and its ancillaries. If used, MEHQ may be added at a rate of1 kg to 15 kg of MEHQ per 10,000 kg of HPMAA heavies column feed stream115; more preferably 1.5 kg to 12 kg of MEHQ per 10,000 kg of HPMAAheavies column feed; most preferably 2 kg to 9 kg of MEHQ per 10,000 kgof HPMAA heavies column feed.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA heavies column 120 and its column ancillaries. Operatingconditions and concerns and recommended oxygen-to-inhibitor ratios forHPMAA heavies column 120 are identical to those described in connectionwith HPMAA lights column 110.

Heavy ends, such as HIBA and other impurities, are removed from thebottom of HPMAA heavies column 120 via line 130. The HPMAA heaviescolumn bottoms stream 130 may be disposed of but fuel values arepreferably recovered before disposal. Optionally, bottoms stream 130 maybe further processed in an independent stripping system to recoverresidual MAA. In one embodiment of an independent stripping system,bottoms stream 130 may be heated in one or more glass-lined strippingvessels with live steam (steam that comes into direct contact with theMAA-containing heavies column bottoms stream). It is preferred that thestripping vessels be operated at sub-atmospheric pressure to maximizethe recovery of MAA. The recovered MAA may be recycled back into a MAAprocess.

To minimize condensation polymerization, vapor spaces on HPMAA heaviescolumn 120 and its ancillaries, including condensers and interconnectingvapor lines, are preferably maintained at a temperature above thedewpoint of MAA; insulation and electric or steam tracing are effectivefor this purpose.

The HPMAA heavies column overhead stream 125 contains a significantamount of MAA as well as water, acetone, and other light ends. Overheadstream 125 may be at least partially condensed. If overhead stream 125is so condensed, tempered water may be used in the condenser(s) to avoidfreezing the MAA in the stream. To maintain the required purity ofstream 165, it is often necessary to return a portion of the condensateback to HPMAA heavies column 120 via reflux line 155; the fraction ofcondensate returned may vary from 0% to 100%, depending on the operatingconditions of HPMAA heavies column 120 and the MAA purity level desired.In a preferred embodiment, a portion of the condensate may berecirculated back to the condenser(s) and optionally to the vapor inletline, to minimize fouling and improve condenser efficiency. Thecondensate may flow freely out of the recirculation line or may beapplied to the tubesheet, condenser interior surfaces, and/or inletvapor line interior walls. If inhibitor is added to the condenser(s), itmay be added through this condensate recirculation stream to improve thedistribution of the inhibitor. In an especially preferred embodiment, atleast a portion of this condensate recirculation stream may pass throughan apparatus that sprays the condensate on the interior surfaces ofHPMAA heavies column 120 and/or its ancillaries to wash offpolymerizable condensates. The remaining condensate, comprising MAA andlight end impurities, is then transferred via line 165 to HPMAAfinishing column 140.

The remaining water, acetone, and other light ends are removed fromHPMAA finishing column 140 via overhead stream 135. A partial-condenserarrangement is preferred, wherein overhead stream 135 may be partiallycondensed into a liquid. If overhead stream 135 is so condensed,tempered water may be used in the condenser(s) to avoid freezing MAA inthe stream. To minimize condensation polymerization, vapor spaces onHPMAA finishing column 140 and its ancillaries, including condensers andinterconnecting vapor lines, are preferably maintained at a temperatureabove the dewpoint of MAA; insulation and electric or steam tracing areeffective for this purpose. In a preferred embodiment, a portion of thecondensate may be recirculated back to the condenser(s) and optionallyto the vapor inlet line, to minimize fouling and improve condenserefficiency. The condensate may flow freely out of the recirculation lineor may be applied to the tubesheet, condenser interior surfaces, and/orinlet vapor line interior walls. If inhibitor is added to thecondenser(s), it may be added through this condensate recirculationstream to improve the distribution of the inhibitor. In an especiallypreferred embodiment, at least a portion of this condensaterecirculation stream may pass through an apparatus that sprays thecondensate on the interior surfaces of HPMAA finishing column 140 and/orits ancillaries to wash off polymerizable condensates.

HPMAA product stream 150 exits HPMAA finishing column 140 from thebottom portion of the column having purity levels greater than or equalto 99% and containing less than 0.05% water. It is preferred that HPMAAstream 150 be cooled before storage to inhibit polymerization. In someinstances, polymer or other undesirable components may be present in thestream; therefore, it may be desirable to filter HPMAA product stream150 to remove any traces of polymer or other undesirable componentsprior to storage. HPMAA finishing column 140 and its column ancillariesare preferably constructed of corrosion resistant material, as describedabove for HPMAA lights column 110. Internal components such as trays orpacking may be used in HPMAA finishing column 140, if desired.Internals, if present, may be made from the same materials as HPMAAfinishing column 140 itself or may be constructed from one or moredifferent materials. For example, the upper portion of HPMAA finishingcolumn 140 may contain stainless steel packing, while the lower portionof the column may contain zirconium trays. Perforated plate trays areespecially preferred, as they have been found to be particularlyresistant to MAA polymer accumulation. If trays are used, it ispreferable that two to ten perforated plate trays are used.

Preferably, HPMAA finishing column 140 is operated under a vacuum tominimize the temperature in the bottom of the column. For example, in apreferred embodiment, the pressure at the bottom of the column ismaintained at 50 mmHg to 80 mmHg, allowing the bottom of the column tobe operated between 70° C. to 110° C.

At least one heat exchanger may be used as the heating apparatus for thefinishing column. Desuperheated steam is preferred as the heatexchanger's heat source. If a reboiler is used as the heat exchanger, itmay be internal or external to the distillation column. Vortex breakersare also useful in the bottom of HPMAA finishing column 140.

It is oftentimes useful to add inhibitors, such as those listed above,to HPMAA finishing column 140 with or without a diluent. Such inhibitormay be added in one or more locations throughout HPMAA finishing column140 and its ancillaries. The preferred inhibitor for HPMAA finishingcolumn 140 is MEHQ. It is preferred that the inhibitor is added at arate that does not exceed product inhibitor specifications in the HPMAAproduct stream 150. Optionally, a variable amount of MEHQ inhibitor maybe added directly to the HPMAA product stream 150 to ensure that theHPMAA product stream inhibitor concentration is within final productspecifications.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA finishing column 140 and its column ancillaries. Operatingconditions and concerns and recommended oxygen-to-inhibitor ratios forHPMAA finishing column 140 are identical to those described inconnection with HPMAA lights column 110.

Another embodiment of an HPMAA purification system is shown in FIG. 2that utilizes a side-draw configuration for the HPMAA product stream.For one skilled in the art, the similarities of this embodiment to thesystem depicted in FIG. 1 will be evident. The configuration andfunction of the first two impurity removal apparatus (columns 210 and220) is essentially the same as described in the previous embodiment(columns 110 and 120, respectively). Similarly, columns 210 and 220 andtheir column ancillaries are preferably constructed of corrosionresistant material, as previously described for HPMAA lights column 110.The material exiting column 220 via line 265 is a near-HPMAA qualityintermediate stream; this stream may be further purified in HPMAAfinishing column 240 wherein water and light ends are removed from theupper part of the column through overhead stream 235. In the embodimentof FIG. 2, however, the HPMAA is withdrawn from the side of a thirdimpurity removal apparatus, HPMAA finishing column 240, rather thandrawn from the bottom. The finishing column bottoms stream 245,comprising heavy ends, may be recycled, for example to column 220 (asshown) or optionally to column 210, to maximize MAA yield. HPMAAfinishing column 240 and its column ancillaries are preferablyconstructed of corrosion resistant material, as previously described forHPMAA lights column 110.

Water, acetone, and other light ends are removed from HPMAA finishingcolumn 240 via overhead stream 235. A partial-condenser arrangement ispreferred, wherein overhead stream 235 is partially condensed into aliquid. If overhead stream 235 is so condensed, tempered water may beused in the condenser(s) to avoid freezing the MAA in the stream. Tominimize condensation polymerization, the HPMAA finishing column 240 andits ancillaries, including condensers and interconnecting vapor lines,are preferably maintained at a temperature above the dewpoint of MAA;insulation and electric or steam tracing are effective for this purpose.When trays are used in HPMAA finishing column 240, perforated platetrays are preferred, as they have been found to be particularlyresistant to MAA polymer accumulation; two to ten perforated plate traysare especially preferred. In a preferred embodiment, a portion of thecondensate may be recirculated back to the condenser(s) and optionallyto the vapor inlet line, to minimize fouling and improve condenserefficiency. The condensate may flow freely out of the recirculation lineor may be applied to the tubesheet, condenser interior surfaces, and/orinlet vapor line interior walls. If inhibitor is added to thecondenser(s), it may be added through this condensate recirculationstream to improve the distribution of the inhibitor. In an especiallypreferred embodiment, at least a portion of this condensaterecirculation stream may pass through an apparatus that sprays thecondensate on the interior surfaces of HPMAA finishing column 140 and/orits ancillaries to wash off polymerizable condensates.

HPMAA product stream 250 exits HPMAA finishing column 240 from the sideof the column having purity levels greater than or equal to 99% andcontaining less than 0.05% water. Product stream 250 is preferablycooled before storage to inhibit polymerization. Removal of the productstream from the side of the column (known herein as a “side-draw”configuration), instead of from the bottom of the column, allows forimproved operation of HPMAA finishing column 240. Because the highesttemperature occurs at the bottom of the column, polymer or otherundesirable impurities may form and be present in HPMAA product drawndirectly from the bottom of the column. While optional filtration ofproduct stream 250 may be used as described in the previous embodimentdepicted in FIG. 1, the use of the side-draw configuration in thisembodiment may reduce the level of potential impurities in the HPMAAproduct stream. Thus, the need for filtration may be minimized and thecost of operation reduced, providing an advantage for the manufacturer.

As shown in FIG. 2, heavy end impurities, which may accumulate in thebottom of HPMAA finishing column 240, are removed via stream 245 andrecycled back to HPMAA heavies column 220. This recycle step allows theMAA present in stream 245 to be recovered as product in column 220. Anyheavy ends and undesirable impurities present in stream 245 will exitcolumn 220 with the other heavy end components in stream 230. It shouldbe noted that, while heavy ends stream 245 may alternatively be recycledback to lights column 210, this step is functionally equivalent to theembodiment shown in FIG. 2 and, as such, will provide similar benefitsto the producer.

It is oftentimes useful to add inhibitors, such as those listed above,to HPMAA finishing column 240, optionally with a diluent. Inhibitor maybe added in one or more locations throughout HPMAA finishing column 240and its ancillaries.

The side-draw removal of the HPMAA product stream increases inhibitoroptions for HPMAA finishing column 240. This is due to the fact thatinhibitors are generally heavy components that exit the distillationcolumn through the bottoms. Thus, when the product stream is drawn fromthe bottom of the column any added inhibitor exits along with it. By wayof contrast, when the product is drawn from the side of the column, allof the inhibitor is not drawn off with the product, rather the inhibitordrops to the bottom of the column for removal. Thus, in the embodimentillustrated in FIG. 1 only those inhibitors which are within the finalproduct specification may be used in HPMAA finishing column 140;whereas, in the embodiment illustrated in FIG. 2 a wider variety ofinhibitors may be employed in HPMAA finishing column 240.

PTZ is particularly useful for minimizing polymer formation in thebottoms of column 240 and is preferred. If used, PTZ is preferably added(optionally with a diluent) at a rate of 0.005 kg to 8 kg of PTZ per10,000 kg of HPMAA finishing column 240 feed; more preferably 0.01 kg to5 kg of PTZ per 10,000 kg of HPMAA finishing column 240 feed; mostpreferably 0.05 kg to 1 kg of PTZ per 10,000 kg of HPMAA finishingcolumn 240 feed.

If HQ inhibitor is used, it is preferred that the inhibitor be added(optionally with a diluent) at a rate of 1 kg to 10 kg of HQ per 10,000kg of HPMAA finishing column 240 feed; more preferably 1.3 kg to 8 kg ofHQ per 10,000 kg of HPMAA finishing column 240 feed; most preferably 1.5kg to 5 kg of HQ per 10,000 kg of HPMAA finishing column 240 feed.

MEHQ inhibitor may also be added to HPMAA finishing column 240 in thisembodiment and may be added directly, or with a diluent such as MAA,throughout HPMAA finishing column 240 and its ancillaries. Because ofthe side-draw configuration of HPMAA finishing column 240, it ispossible to use higher MEHQ inhibitor addition rates than in HPMAAfinishing column 140. In general, satisfactory performance will beachieved in HPMAA finishing column 240 if the MEHQ addition rates forHPMAA finishing column 140, as described above, are utilized.Optionally, a variable amount of MEHQ inhibitor may be added directly tothe HPMAA product stream 250 to ensure that the HPMAA product streaminhibitor concentration is within final product specifications.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA finishing column 240 and its column ancillaries. Operatingconditions and concerns and recommended oxygen-to-inhibitor ratios forHPMAA finishing column 240 are identical to those described inconnection with HPMAA lights column 110.

Another embodiment of an HPMAA purification system is shown in FIG. 3.In this embodiment, three impurity removal apparatus are utilized topurify crude MAA stream 300 to HPMAA. Crude MAA stream 300 is initiallyfed to the first of three impurity removal apparatus, HPMAA heaviescolumn 310. In HPMAA heavies column 310, heavy ends, including HIBA, areremoved first from the bottom of the column via line 315. Early removalof heavies in this embodiment prevents HIBA decomposition to water andlight ends in the next two columns.

HPMAA heavies column 310 and its column ancillaries are preferablyconstructed of corrosion resistant material, as previously described forHPMAA light column 110. Internal components such as trays or packing maybe used in HPMAA heavies column 310, if desired. Internals, if present,may be made from the same materials as the column itself or may beconstructed from one or more different materials. Trays are preferred inHPMAA heavies column 310. Perforated plate trays are especiallypreferred, as they have been found to be particularly resistant to MAApolymer accumulation. If trays are used, it is preferable that five tofifteen perforated plate trays are used. It is preferred that HPMAAheavies column 310 be operated under a vacuum to minimize thetemperature of the bottom of the column. For example, in a preferredembodiment, the pressure at the bottom of the column is maintained at 60mmHg to 100 mmHg, allowing the bottom of the column to be operated at75° C. to 115° C. Preferably, at least one heat exchanger may be used asthe heating apparatus for HPMAA heavies column 310. Desuperheated steamis preferred as the heat exchanger's heat source. If a reboiler is usedas the heat exchanger, it may be internal or external to the column.Vortex breakers are also useful in the bottom of HPMAA heavies column310.

It is oftentimes useful to add inhibitors such as those listed above,with or without diluents, to HPMAA heavies column 310. HQ inhibitor isespecially preferred and may be added directly, or with a diluent suchas water, in one or more locations throughout HPMAA heavies column 310and its ancillaries. If used, it is preferred that the inhibitor beadded at a rate of 1 kg to 10 kg of HQ per 10,000 kg of HPMAA heaviescolumn feed; more preferably 1.3 kg to 8 kg of HQ per 10,000 kg of HPMAAheavies column feed; most preferably 1.5 kg to 5 kg of HQ per 10,000 kgof HPMAA heavies column feed.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA heavies column 310 and its column ancillaries. Operatingconditions and concerns and recommended oxygen-to-inhibitor ratios forHPMAA heavies column 310 are identical to those described in connectionwith HPMAA lights column 110.

HIBA, other heavy ends, and impurities are removed from the bottom ofthe heavies column via line 315 and it may be disposed of or recoveredfor fuel values. Optionally, the heavies column bottoms can be furtherprocessed in an independent stripping system to recover residual MAA. Inone embodiment of an independent stripping system, the heavies columnbottoms are heated in one or more glass-lined stripping vessels withlive steam. It is preferred that the stripping vessels be operated atsub-atmospheric pressure to maximize the recovery of MAA.

The HPMAA heavies column overhead stream 305 contains a significantamount of MAA as well as water, acetone, other light ends, and traceamounts of heavy ends. HPMAA heavies column overhead stream 305 ispreferably at least partially condensed.

To maintain the required purity of the stream 365, it is often necessaryto return a portion of the condensate back to the heavies column viareflux line 355; the fraction of condensate returned may vary from 0% to100%, depending on the operating conditions of HPMAA heavies column 310and the MAA purity level desired. The remaining condensate is thentransferred via line 365 to a second impurities removal apparatus, HPMAAlights column 320. Tempered water may be used in the heavies columncondenser(s) to avoid freezing MAA in the stream. To minimizecondensation polymerization, vapor spaces on HPMAA heavies column 310and its ancillaries, including condensers and interconnecting vaporlines, are preferably maintained at a temperature above the dewpoint ofMAA; insulation and electric or steam tracing are effective for thispurpose. In a preferred embodiment, a portion of the condensate may berecirculated back to the condenser, and optionally to the vapor inletline, to minimize fouling and improve condenser efficiency. Thecondensate may flow freely out of the recirculation line or may beapplied to the tubesheet, condenser interior surfaces, and/or inletvapor line interior walls. If inhibitor is added to the condenser, itmay be added to this condensate recirculation stream to improve thedistribution of the inhibitor. In an especially preferred embodiment, atleast a portion of this condensate recirculation stream may pass throughan apparatus that sprays the condensate on the interior surfaces ofHPMAA heavies column 310 and/or its ancillaries to wash offpolymerizable condensates.

HPMAA lights column 320 removes water, acetone, and other lightimpurities from the MAA via stream 325. HPMAA lights column 320 and itscolumn ancillaries are preferrably constructed of corrosion resistantmaterial, as previously described for HPMAA lights column 110. Internalcomponents such as trays or packing may be used in HPMAA lights column320, if desired. Internals, if present, may be made from the samematerials as the column itself or may be constructed from one or moredifferent materials. Perforated plate trays are especially preferred, asthey have been found to be particularly resistant to MAA polymeraccumulation. If trays are used, it is preferable that two to tenperforated plate trays are used. It is preferred that HPMAA lightscolumn 320 be operated under a vacuum to minimize the temperature at thebottom of the column. For example, in a preferred embodiment, thepressure at the bottom of the column is maintained at 50 mmHg to 80mmHg, allowing the bottom of the column to be operated at 70° C. to 110°C. At least one heat exchanger may be used as the heating apparatus forHPMAA lights column 320. Desuperheated steam is preferred as the heatexchanger's heat source. If a reboiler is used as the heat exchanger, itmay be internal or external to the column. Vortex breakers are alsouseful in the bottom of HPMAA lights column 320.

It is oftentimes useful to add one or more inhibitors, such as thoselisted above, to HPMAA lights column 320. The inhibitors may be added tothe column directly, or with a diluent, in one or more locationsthroughout HPMAA lights column 320 and its ancillaries. PTZ isparticularly useful for minimizing polymer formation in the columnbottoms and is preferred.

If PTZ is used in HPMAA lights column 320, it is preferably added(optionally with diluent) at a rate of 0.05 kg to 12 kg of PTZ per10,000 kg of column feed; more preferably 0.1 kg to 10 kg of PTZ per10,000 kg of column feed; and most preferably 0.4 kg to 5 kg of PTZ per10,000 kg of column feed.

If HQ is used in HPMAA lights column 320, it is preferably added(optionally with diluent) at a rate of 1 kg to 10 kg of HQ per 10,000 kgof column feed; more preferably 1.3 kg to 8 kg of HQ per 10,000 kg ofcolumn feed; most preferably 1.5 kg to 5 kg of HQ per 10,000 kg ofcolumn feed.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA lights column 320 and its column ancillaries. Operating conditionsand concerns and recommended oxygen-to-inhibitor ratios for HPMAA lightscolumn 320 are identical to those described in connection with HPMAAlights column 110.

The MAA, acetone, and water are taken off of the top of HPMAA lightscolumn 320 via line 325. A partial-condenser arrangement is preferred,wherein stream 325 is at least partially condensed into a liquid. Ifstream 325 is so condensed, tempered water may be used in thecondenser(s) to avoid freezing MAA in the stream. To minimizecondensation polymerization, vapor spaces on HPMAA lights column 320 andits ancillaries, including condensers and interconnecting vapor lines,are preferably maintained at a temperature above the dewpoint of MAA;insulation and electric or steam tracing are effective for this purpose.In a preferred embodiment, a portion of the condensate may berecirculated back to the condenser and optionally to the vapor inletline, to minimize fouling and improve condenser efficiency. Thecondensate may flow freely out of the recirculation line or may beapplied to the tubesheet, condenser interior surfaces, and/or inletvapor line interior walls. If inhibitor is added to the condenser, itmay be added to this condensate recirculation stream to improve thedistribution of the inhibitor. In an especially preferred embodiment, atleast a portion of this condensate recirculation stream may pass throughan apparatus that sprays the condensate on the interior surfaces ofHPMAA heavies column 320 and/or its ancillaries to wash offpolymerizable condensates.

A high purity MAA stream 330 with a small amount of heavy ends isremoved from the bottom of the lights column and is fed to a third andfinal impurity removal apparatus, HPMAA finishing column 340. In HPMAAfinishing column 340, MAA is separated from the remaining heavy endimpurities to produce HPMAA.

HPMAA finishing column 340 and its column ancillaries are preferablyconstructed of corrosion resistant material, as previously described forHPMAA lights column 110. Internal components such as trays or packingmay be used in HPMAA finishing column 340, if desired. Internals, ifpresent, may be made from the same materials as the column itself or maybe constructed from one or more different materials. Perforated platetrays are especially preferred, as they have been found to beparticularly resistant to MAA polymer accumulation. If trays are used,it is preferable that five to fifteen perforated plate trays are used.HPMAA finishing column 340 is preferably operated such that thedecomposition of any remaining trace amounts of HIBA is avoided.Preferably, HPMAA finishing column 340 is operated under a vacuum (i.e.,below atmospheric pressure) to minimize bottoms temperature. Forexample, in a preferred embodiment, the pressure at the bottom of HPMAAfinishing column 340 is maintained at 60 mmHg to 100 mmHg, allowing thebottom of HPMAA finishing column 340 to be operated at 75° C. to 115° C.

At least one heat exchanger may be used as the heating apparatus for thefinishing column. Desuperheated steam is preferred as the heatexchanger's heat source. If a reboiler is used as the heat exchanger, itmay be internal or external to the column. Vortex breakers are alsouseful in the bottom of HPMAA finishing column 340.

HPMAA having purity levels greater than or equal to 99% and less than0.05% water leaves HPMAA finishing column 340 via line 335 and is atleast partially condensed. Tempered water may be used in the condenserto avoid freezing MAA in the stream. In order to maintain the requiredpurity of the HPMAA product, it is often necessary to return a portionof the condensate back to HPMAA finishing column 340 via reflux line360; the fraction of condensate returned may vary from 0% to 100%,depending on the operating conditions of HPMAA finishing column 340 andthe HPMAA purity level desired. The remaining condensate, exits viaHPMAA product stream 350 from the top portion of the column havingpurity levels greater than or equal to 99% and containing less than0.05% water. The HPMAA product may be cooled before storage to inhibitpolymerization. To minimize condensation polymerization, vapor spaces onHPMAA finishing column 340 and its ancillaries, including condensers andinterconnecting vapor lines, are preferably maintained at a temperatureabove the dewpoint of MAA. Insulation and electric or steam tracing aresuitable for this purpose. In a preferred embodiment, a portion of HPMAAfinishing column 340 condensate may be recirculated back to thecondenser, and optionally to the vapor inlet line, to minimize foulingand improve condenser efficiency. The condensate may flow freely out ofthe recirculation line or may be applied to the tubesheet, condenserinterior surfaces, and/or inlet vapor line interior walls. If inhibitoris added to the condenser, it may be added to this condensaterecirculation stream to improve the distribution of the inhibitor. In anespecially preferred embodiment, at least a portion of this condensaterecirculation stream may pass through an apparatus that sprays thecondensate on the interior surfaces of HPMAA finishing column 340 and/orits ancillaries to wash off polymerizable condensates.

HIBA and other impurities are removed from the bottom of HPMAA finishingcolumn 340 via line 345. The recovery of the MAA is maximized byrecycling this stream to HPMAA heavies column 310.

It is oftentimes useful to add one or more inhibitors, such as thoselisted above, to HPMAA finishing column 340, optionally with a diluent.Such inhibitor may be added in one or more locations throughout HPMAAfinishing column 340 and its ancillaries. PTZ is particularly useful forminimizing polymer formation in the bottom of HPMAA finishing column 340and is preferred. If used, PTZ is preferably added (optionally withdiluent) at a rate of 0.005 kg to 8 kg of PTZ per 10,000 kg of HPMAAfinishing column 340 feed; more preferably 0.01 kg to 5 kg of PTZ per10,000 kg of HPMAA finishing column 340 feed; most preferably 0.05 kg to1 kg of PTZ per 10,000 kg of HPMAA finishing column 340 feed. If HQ isused, it is preferred that the inhibitor be added at a rate from 1 kg to10 kg of HQ per 10,000 kg of HPMAA finishing column 340 feed; morepreferably 1.3 kg to 8 kg of HQ per 10,000 kg of HPMAA finishing column340 feed; most preferably 1.5 kg to 5 kg of HQ per 10,000 kg of HPMAAfinishing column 340 feed.

MEHQ may also be added to HPMAA finishing column 340 directly, or with adiluent such as MAA, in one or more locations in HPMAA finishing column340 and its associated equipment. If MEHQ is used, it is preferred thatthe inhibitor be added at a rate from 1 kg to 15 kg of MeHQ per 10,000kg of feed stream 330. Because the HPMAA product is taken overhead,however, it is not critical to restrict the MEHQ inhibitor addition rateto HPMAA finishing column 340 to this range. To one skilled in the artit will be apparent that, because the HPMAA is taken overhead, it may bepossible to make a product within the required specifications even ifthe preferred inhibitor addition rates are exceeded; however, operationexceeding preferred inhibitor rates will be inefficient.

If more than one inhibitor is introduced directly into HPMAA finishingcolumn 340, the addition rates of the individual inhibitors may bereduced relative to the rates disclosed above. Regardless of theinhibitors used in the finishing column and their respective additionrates, a variable amount of MEHQ inhibitor may be added directly tostream 350 to ensure that the HPMAA product stream inhibitorconcentration is within final product specifications.

As described above, when phenolic inhibitors, such as HQ and MEHQ, areused, it is further preferred that oxygen-containing gas be added to thedistillation column to enhance the effectiveness of the inhibitor.Oxygen-containing gas may be added in one or more locations throughoutHPMAA finishing column 340 and its column ancillaries. Operatingconditions and concerns and recommended oxygen-to-inhibitor ratios forHPMAA finishing column 340 are identical to those described inconnection with HPMAA lights column 110.

By way of example, and not limitation, the following description,relating to the operation of the HPMAA purification system that iswithin the scope of this invention, is provided to illustrate the use ofthe inhibitor in conjunction with an oxygen-containing gas:

EXAMPLE 1

A crude MAA feed stream, comprising greater than 80% MAA, is fed toHPMAA lights column 110 at a rate of 4,545 kg/hr. The pressure at thebottom of the column is 65 mm Hg and the temperature at the bottom ofthe column is maintained at 90° C. to 100° C. Inhibitor solutioncomprising 3.5% HQ in water is added in multiple locations throughoutthe HPMAA lights column and its ancillaries to yield an overall solutionfeed rate of 23 kg/hr. Atmospheric air is added to the reboilercirculation line at a rate of 5 kg/hr. The resultant ratio ofoxygen-containing gas addition to inhibitor is 4.5 moles O₂ per mole ofHQ, and polymer formation in the distillation column is effectivelyinhibited.

EXAMPLE 2

An MAA feed stream, comprising greater than 90% MAA, is fed to the HPMAAheavies column 120 at a rate of 9,090 kg/hr. The pressure at the bottomof the column is 60 mm Hg and the temperature at the bottom of thecolumn is maintained at 100° C. to 105° C. Inhibitor solution comprising2.5% MEHQ in GMAA is added in multiple locations throughout the HPMAAheavies column and its ancillaries to yield an overall solution feedrate of 126 kg/hr. Atmospheric air is added to the reboiler circulationline at a rate of 9 kg/hr. The resultant ratio of oxygen-containing gasaddition to inhibitor is 2.6 moles O₂ per mole of MEHQ, and polymerformation in the distillation column is effectively inhibited.

The present invention, therefore, is well adapted to carry out theobjects and attain both the ends and the advantages mentioned, as wellas other benefits inherent therein. While the present invention has beendepicted, described, and is defined by reference to particularembodiments of the invention, such references do not imply a limitationon the invention, and no such limitation is to be inferred. Theinvention is capable of considerable modification, alteration, andsubstitution of equivalents in form and/or function, as will occur tothose of ordinary skill in the pertinent arts. The depicted anddescribed embodiments of the invention are exemplary only, and are notexhaustive of the scope of the invention. Consequently, the invention isintended to be limited only by the spirit and scope of the appendedclaims, giving full cognizance to equivalents in all respects.

1-10. (canceled)
 11. An apparatus for the preparation of high puritymethacrylic acid, said apparatus comprising at least one distillationcolumn and its ancillaries, said at least one distillation column havinga bottom and comprising perforated plate trays, said at least onedistillation column and its ancillaries being designed to operate in therange of from 50 mmHg to 80 mmHg, allowing said bottom to be operated ata temperature from 70° C. to 100° C., said at least one distillationcolumn and its ancillaries being constructed, at least in part, of acorrosion resistant material selected from the group consisting of 300series stainless steel, 904L stainless steel, 6-moly stainless steel,Hastelloy®, tantalum, zirconium and covered base metal. 12-13.(canceled)
 14. An apparatus for the preparation of high puritymethacrylic acid, said apparatus comprising at least one distillationcolumn and its ancillaries, said at least one distillation column havinga bottom and comprising perforated plate trays, said at least onedistillation column and its ancillaries being designed to operate in therange of from 60 mmHg to 100 mmHg, allowing said bottom to be operatedat a temperature from 75° C. to 115° C., and said at least onedistillation column and its ancillaries being constructed, at least inpart, of a corrosion resistant material selected from the groupconsisting of 300 series stainless steel, 904L stainless steel, 6-molystainless steel, Hastelloy®, tantalum, zirconium and covered base metal,wherein said at least one distillation.